Universal conductance fluctuations in Cu:Mn nano-contacts of different Mn concentrations c yield the phase coherence length L_f which exhibits a minimum at c 100 ppm. Above this concentration which marks the onset of a spin-glass state below 0.2 K, L_f depends on the magnetic field range, rising to nearly the c=0 value for B in [9.5, 12.5 T] and leveling-off at a much lower value for B in [0, 3 T]. These results demonstrate the recovery of phase coherence due to spin-glass correlations and a strong magnetic field.